An automatic voltage regulator for a separately excited generator is well known which is adapted to control an exciting current in response to the output voltage of the generator. It has been disclosed in British Pat. No. 966,024 issued on Aug. 6, 1964, for example. One of the disadvantages of such a prior art is that an exciting voltage from an output of an exciter irregularily varies due to separate characteristics of two controlled rectifying means incorporated between the exciter and the exciting winding of the generator with the result that the output voltage of the generator tends to be distorted. Such distorted output voltage of the generator is shown at dotted line in FIG. 3C wherein the output voltage is shown to be conveniently rectified. Variation indicated by .DELTA.E in FIG. 3C causes a lighting device to flicker if it is operatively associated with the output of the generator. Another disadvantage of the prior art is that the regulator cannot regulate the output voltage of the generator in a minute manner because the operating frequency of the exciter is generally very small so that it approximately equals the operating frequency of the generator.